我是WPF的新手。我有一个使用Entity框架工作从数据库生成的Country类。当我按pageup或pageDown键导航到Country类以显示Country id并命名下一个元素的第一个到最后一个元素。当我运行代码时，我得到了Exception，请告诉我如何导航3个表 我的数据库类：
公共部分国家{
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
public Country()
{
this.PurchaseOrderLineItems = new HashSet<PurchaseOrderLineItem>();
this.States = new HashSet<State>();
}
public int CountryID { get; set; }
public string Country_Name { get; set; }
public string Country_Code { get; set; }
public Nullable<System.DateTime> CreatedDate { get; set; }
public Nullable<int> CreatedBy { get; set; }
public Nullable<System.DateTime> LastUpdatedDate { get; set; }
public Nullable<int> LastUpdatedBy { get; set; }
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
public virtual ICollection<PurchaseOrderLineItem> PurchaseOrderLineItems { get; set; }
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
public virtual ICollection<State> States { get; set; }
}
导航Businesslogic类是：
public Country NavigateCoutryCodebyID（int countryCodeID，NavigationType nav） {
//.Include("State").Include("PurchaseOrderLineItem")
Country countryCode = null;
try
{
using (LWLLCDBEntities dbEntities = new LWLLCDBEntities())
{
//current item
switch (nav)
{
case NavigationType.First:
countryCode = dbEntities.Countries.FirstOrDefault();
break;
case NavigationType.Last:
countryCode = dbEntities.Countries.OrderByDescending(t => t.CountryID).FirstOrDefault();
break;
case NavigationType.Current:
countryCode = dbEntities.Countries.Where(i => i.CountryID == countryCodeID).FirstOrDefault();
break;
case NavigationType.Next:
countryCode = dbEntities.Countries.Where(i => i.CountryID > countryCodeID).FirstOrDefault();
if (countryCode == null)
countryCode = dbEntities.Countries.FirstOrDefault();
break;
case NavigationType.Prev:
countryCode = dbEntities.Countries.Where(i => i.CountryID < countryCodeID).FirstOrDefault();
if (countryCode == null)
countryCode = dbEntities.Countries.OrderByDescending(t => t.CountryID).FirstOrDefault();
break;
}
}
}
catch (Exception ex)
{
if (log.IsErrorEnabled)
{
log.Error("BusinessLogic.NavigateCountryCodebyID in Business Logic threw exception: ", ex);
}
}
return countryCode;
}